A touchpad is an input device for an object such as a finger and a touch pen to slide or touch thereon. One of its applications is to generate a signal to control a cursor on a display screen to move with change of relative coordinates or absolute coordinates. Moreover, a touchpad may support other extended functions such as button simulation. Due to the recent significant progress in technologies of displays and graphic chips, computer systems and even family multimedia systems now support high-definition displays, yet touchpads limited to their physical size are unable to support commensurate definition. For providing more convenient using environment, the so-called “edge-motion” function has been developed, by which when an object moves to an edge of a touchpad, the cursor on a window will automatically move or the page on a window will automatically scroll. For example, U.S. Pat. No. 5,880,411 proposes a method for a touchpad, by which, when an object slides to an edge of the touchpad, an automatic movement signal for a cursor will be asserted with a fixed direction pre-defined for the edge. However, this method only generates an automatic movement signal with either of four pre-defined directions, i.e. up, down, left and right, for the four edges of a touchpad, and thus lacks of flexibility in design.